Table 3. Determination of the camera constant (the calibrated focal length). 
  
  
  
  
  
  
  
| Camera | Eagle III Eagle III RMK P10 RC: 5+ 
en SEA gl Eo RTC — 18° C Topogon Aviotar | 
Experiment | mm mm mm mm | 
1 126.35 126.32 99.38 209.36 | 
2 | 126.37 126.37 99.36 209.36 | 
3 | 126.35 126.37 99.26 209.35 | 
Averages ........ |... 126.36 126.36 99.38 209.86 | 
Tah det... sls | (126.63) (126.63) (99.56) (209.45) | 
Stand. error ......] 0.006 0.006 0.005 | 0.010 | 
(1.0s, ) | | 
  
The standard error of the calibrated focal lengths can be determined 
as follows. 
C 
We have the relation de = — dr 
Hences = — s 
But s is under present circumstances for the five point combina- 
tion, see [3], about 9. where s, is the standard error of unit weight of 
2 
the image coordinate measurements. Consequently 
C 
S E. 
2r 
Evidently the accuracy of the determination of the calibrated focal 
length is dependent upon the radius r of the zero-point of the distortion 
: C 
curve, Assuming r = — we find s, — s, 
2 
The principal point of the photograph can preliminarily be indicated 
by the image of the center point on the ground, provided that the ca- 
mera is carefully levelled and adjusted above the actual point. 
From the computations however, small corrections dx, and dy, of 
the camera will be obtained, which can be expressed as corrections to 
the preliminary position of the principal point. For each circle one set 
of corrections will be obtained. The accuracy of these corrections will 
vary from circle to circle and can be expressed with the aid of the cor- 
responding weight numbers. 
The final values of the corrections can be determined as weighted 
means. The standard error of the mean values can then be determined 
in the usual way. 
24